The invention relates to anodes for use in light-weight rechargeable electrochemical cells, and to such cells. The invention is illustrated in the following with reference to lithium and lithium-alloy anode cells. It ought to be understood that the invention also relates to other anodes and electrochemical cells, wherein the system exhibits essentially the same unique characteristics as that of lithium anodes of the invention.
Lithium metal has one of the lowest equivalent weights and its inclusion and recycling to an extent of 5 to 20 percent of electrode weight gives one of the lowest weights possible for negative, secondary cell electrodes. As such it is important for energy storage in such uses which require low weight per energy unit.
Pure lithium cannot be deposited from non-aqueous solution without the formation of a protecting film on the electrode. This film is known as Solid Electrolyte Interphase, and this inhibits the passage of electrons from the metal electrode to the electrolyte solution. Thermodynamically the reduction reactions of the electrolyte are preferable to the deposition of pure lithium. On the other hand, deposition of lithium in the form of an intermetallic compound or intermediate phase-like metallic solution may be preferable to some of these reduction reactions. If the lithium deposition is not quick enough, large overpotentials may be needed, making a quicker electrolyte reduction reaction kinetically preferable.
Reduction of electrolyte solution during deposition of free lithium results also in shortening of battery life, because some of the reduction products are insoluble in the solution and thus precipitate on the surface of the lithium anode.
In the case of pure lithium deposition, the isolation of active lithium masses from each other and from the current collector, is a common disadvantage.
Another failure mechanism of rechargeable lithium batteries is shorts formation by lithium dendrites leading sometimes to the explosion of such batteries.
In the prior art, lithium alloy electrodes, based either on single or mixed matrix models were used. Two major drawbacks have been encountered:
1) The rechargeability of Lithium s limited to only 3-6% Wt of the active mass (ref 1). PA1 2) crumbling of the anode into powder during use. Furthermore iron or nickel were added (ref.2) in high proportions to give better performance, reducing even more the weight percentage of the lithium. Several Examples from the literature for compositions that have been tried, are: PA1 As, P, Si, Ge, C, Fe. Ni, Cu, Cr, V, Co, Zn, Mo, Nb, Mn.
______________________________________ (w/w%-approximate): ______________________________________ Li7Al93 (crumbles) Li (7%) Al (93%) Li7Sn93 (crumbles) Li (7%) Sn (93%) Li3.5Pb67.5Cd29 (low in Li) Li (3.5%) Pb (67.5%) Cd (29%). ______________________________________